Video coding

ABSTRACT

A method of video encoding including receiving a video signal to be coded; coding data representing a frame of said video signal; and repeating part, but not all, of the data. The repeated part including the picture header for the frame. A method of decoding an encoded video signal including receiving coded data representing frames of a video signal; examining the coded data to detect header data and picture data; when an error in the picture header is detected, storing the picture data in a temporary picture data store, detecting a repeat of the header data; and decoding the stored picture data using the repeated header data.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

UNIK Priority Application 9916909.6, filed Jul. 19, 1999 including thespecification, drawings, claims and abstract, is incorporated herein byreference in its entirety. This application is a Divisional of U.S.application Ser. No. 11/099,596, which is a Continuation of U.S.application Ser. No. 09/619,660, filed Jul. 19, 2000, now U.S. Pat. No.7,006,576, each of which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

This invention relates to video encoding and decoding.

One of the recent targets in mobile telecommunications has been toincrease the speed of data transmission to enable incorporation ofmultimedia services to mobile networks. One of the key components ofmultimedia is digital video. Transmission of video comprises a fairlycontinuous traffic of data representing moving pictures. As is generallyknown, the amount of data needed to transfer pictures is high comparedwith many other types of media, and so far usage of video in lowbit-rate terminals has been negligible. However, significant progresshas been achieved in the area of low bit-rate video compression.Acceptable video quality can be obtained at bit-rates around 20 kilobits per second. As a result of this progressive reduction in bit-rate,video will be a viable service to offer over channels such as mobilecommunications channels.

A video sequence consists of a series of still images or frames. Videocompression methods are based on reducing the redundancy andperceptually irrelevant parts of video sequences. The redundancy invideo sequences can be categorised into spatial, temporal and spectralredundancy. Spatial redundancy means the correlation betweenneighbouring pixels within a frame. Temporal redundancy means thecorrelation between areas of successive frames. Temporal redundancyarises from the likelihood of objects appearing in a previous image alsoappearing in the current image. Compression can be achieved bygenerating motion compensation data which describes the motion (i.e.displacement) between similar areas of the current and a previous image.The current image is thus predicted from the previous one. Spectralredundancy means the correlation between the different colour componentsof the same image.

Video compression methods typically differentiate between images whichdo or do not utilise temporal redundancy reduction. Compressed imageswhich do not utilise temporal redundancy reduction methods are usuallycalled INTRA or I-frames whereas temporally predicted images are calledINTER or P-frames (and also B-frames when the INTER frames may bepredicted in a forward or backward manner). In the INTER frame case, thepredicted (motion-compensated) image is rarely precise enough andtherefore a spatially compressed prediction error image is also a partof each INTER frame.

However, sufficient compression cannot usually be achieved by justreducing the redundancy of the video sequence. Thus, video encoders tryto reduce the quality of those parts of the video sequence which aresubjectively the least important. In addition, the redundancy of theencoded bitstream is reduced by means of efficient lossless coding ofcompression parameters and coefficients. The main technique is to usevariable length codes.

Compressed video is easily corrupted by transmission errors, mainly fortwo reasons. Firstly, due to utilisation of temporal predictivedifferential coding (INTER frames), an error is propagated bothspatially and temporally. In practice this means that, once an erroroccurs, it is easily visible to the human eye for a relatively longtime. Especially susceptible are transmissions at low bit-rates wherethere are only a few INTRA-coded frames (the transmission of INTRA-codedframes would terminate the temporal error propagation). Secondly, theuse of variable length codes increases the susceptibility to errors.When a bit error alters a codeword to another one of different length,the decoder loses codeword synchronisation and also decodes subsequenterror-free codewords (comprising several bits) incorrectly until thenext synchronisation or start code. (A synchronisation code is a bitpattern which cannot be generated from any legal combination of othercodewords.)

One of the inherent characteristics of wireless data transmission is arelatively high bit error probability. This problem can be addressed byvarious transport, network and link layer retransmission schemes.However the drawback of such schemes is the possibility of unlimited andfluctuating transmission delays. In conversational audio-visualservices, it is unacceptable to have large end-to-end delays. Thusretransmission schemes cannot be used in such services. Instead one musttry to detect and conceal the transmission errors. In streamingaudio-visual retrieval services, the transmission delay may varysomewhat due to the fact that some initial buffering occurs before thestart of play-back. However, the maximum acceptable transmission delayis fixed and, if exceeded, there is an annoying pause in the play-back.In practice, both reliable and unreliable transport channels are used inretrieval services.

Every bit in a compressed video bitstream does not have an equalimportance to the decompressed images. Some bits define vitalinformation such as picture type (e.g. INTRA or INTER), quantiser valueand optional coding modes that have been used. ITU-T RecommendationH.263 relates to video coding for low bit-rate communication. In H.263,the most vital information is gathered in the picture header. Atransmission error in the picture header typically causes a totalmisinterpretation of the subsequent bits defining the picture content.Due to utilisation of temporal predictive differential coding (INTERframes), the error is propagated both spatially and temporally. Thus, anormal approach to picture header corruption is to freeze the previouspicture on the screen, to send an INTRA picture request to thetransmitting terminal and to wait for the requested INTRA frame. Thismay cause an annoying pause in the received video, especially inreal-time conversational video sequences.

Transmission errors have a different nature depending on the underlyingnetwork. In packet-switched networks, such as the internet etc.,transmission errors are typically packet losses (due to congestion innetwork elements). In circuit-switched networks, such as mobile networks(e.g. HSCSD for GSM), transmission errors are typically bit errors wherea ‘1’ is corrupted to ‘0’ or vice versa.

To impede degradations in images introduced by transmission errors,retransmissions can be used, error detection and/or error correctionmethods can be applied, and/or effects from the received corrupted datacan be concealed. Normally retransmission provides a reasonable way toprotect video data streams from errors, but large round-trip delaysassociated with low bit-rate transmission and moderate or high errorrates make it practically impossible to use retransmission, especiallywith real-time videophone applications. Error detection and correctionmethods usually require large transmission overheads since they add someredundancy to the data. Consequently, for low bit-rate applications,error concealment can be considered as a preferred way to protect andrecover images from transmission errors. Video error concealment methodsare typically applicable to transmission errors occurring through packetloss and bit corruption.

H.263 is an ITU-T recommendation of video coding for low bit-ratecommunication which generally means data rates below 64 kbps. Therecommendation specifies the bitstream syntax and the decoding of thebitstream. Currently, there are two versions of H.263. Version 1consists of the core algorithm and four optional coding modes. H.263version 2 is an extension of version 1 providing twelve new negotiablecoding modes. H.263 is currently one of the most-favoured coding methodsproposed for mobile wireless applications, where the bit rate is of theorder of 28.8 bits per second and Quarter Common Intermediate Format(QCIF) pictures of 176×144 pixels are usually used. Currently theexpected bit rates for third generation wireless products is around 64kbps and the image resolution may be higher.

Pictures are coded as luminance (Y) and two colour difference(chrominance) components (CB and CR). The chrominance pictures aresampled at half the resolution of the luminance picture along bothco-ordinate axes. Picture data is coded on a block-by-block basis, eachblock representing 8×8 pixels of luminance or chrominance.

Each coded picture, as well as the corresponding coded bitstream, isarranged in a hierarchical structure with four layers, which are frombottom to top: block layer, macroblock layer, picture segment layer andpicture layer. The picture segment layer can either be arranged as agroup of blocks or a slice.

A block relates to 8×8 pixels of luminance or chrominance. Block layerdata consists of uniformly quantised discrete cosine transformcoefficients, which are scanned in zigzag order, processed with arun-length encoder and coded with variable length codes.

Each macroblock relates to 16×16 pixels of luminance and the spatiallycorresponding 8×8 pixels of the two chrominance components. In otherwords, a macroblock consists of four 8×8 luminance blocks and the twospatially corresponding 8×8 colour difference blocks. Each INTERmacroblock is associated with a motion vector which defines the positionof a corresponding area in the reference frame which resembles thepixels of the current INTER macroblock. The INTER macroblock datacomprises coded prediction error data for the pixels of the macroblock.

Usually, each picture is divided into segments known as groups of blocks(GOBs). A group of blocks (GOB) for a QCIF (Quarter Common IntermediateFormat) picture typically comprises one row of macroblocks (i.e. 11macroblocks). Data for each GOB consists of an optional GOB headerfollowed by data for the macroblocks within the GOB.

If the optional slice structured mode is used, each picture is dividedinto slices instead of GOBs. A slice contains a number of consecutivemacroblocks in scan-order. Data for each slice consists of a sliceheader followed by data for the macroblocks of the slice.

The picture layer data contain parameters affecting the whole picturearea and the decoding of the picture data. The coded parameter data isarranged in a so-called picture header. In QCIF format a picture isdivided into 176×144 pixels which corresponds to 9 rows of 11macroblocks.

Picture and GOB (or slice) headers begin with a synchronisation or startcode. No other code word or a legal combination of code words can formthe same bit pattern as the synchronisation codes. Thus, thesynchronisation codes can be used for bitstream error detection and forresynchronisation after bit errors.

H.263 is the video compression standard used in the ITU-T RecommendationH.324“Terminal for Low Bit-Rate Communication” February 1998, whichdefines videophone communication over PSTN and mobile networks. When aH.324 connection is run over a wireless channel, it is likely that thereceived bitstream contains transmission errors. In a H.263 videobitstream, these errors are extremely harmful if they occur in pictureheaders. Such an error may prevent the decoding of the picture contents.Errors in INTRA picture headers cause the most severe implications,since these pictures are used as initial temporal prediction sources.Errors in an INTRA picture header detrimentally affect the correspondingdecoded INTRA picture and each subsequent picture initially predictedfrom this INTRA picture.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of video encodingand decoding as claimed in the appended claims. An encoder and a decoderare also provided as claimed in the appended claims.

A first embodiment of the invention introduces a novel method to repeatINTRA picture headers in video bitstreams, which is fully compliant withthe ITU-T H.263 recommendation. The invention introduces redundantcopies of picture headers in the bitstream. If the primary pictureheader is corrupted, a decoder may use a copy of it to enable thedecoding of the picture contents. This invention introduces an INTRApicture header repetition method that uses the standard syntax andsemantics of H.263. Therefore, all standard compliant decoders canutilise the method.

The inclusion of a repeat of the picture header for at leastINTRA-frames means that a receiving decoder does not necessarily have tofreeze the display, send a repeat request to the encoder and wait forthe encoder to send the repeated information. Thus annoying pauses dueto picture freezing are avoided and an end-user should perceive betterquality video.

The invention is applicable to real-time applications and also tonon-real-time applications, such as retrieval services which may not beable to respond to INTRA repeat requests from a receiving decoder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 shows a multimedia mobile communications system;

FIG. 2 shows an example of the multimedia components of a multimediaterminal;

FIG. 3 shows the typical data structure of a video signal encodedaccording to H.263;

FIG. 4 shows an example of a video codec according to the invention;

FIG. 5 shows the data structure of an encoded video signal output by anencoder according to a first embodiment of the invention;

FIG. 6 shows the data structure of an encoded video signal output by anencoder according to a second embodiment of the invention; and

FIG. 7 is a flow diagram showing the operation of a video encoderaccording to a third embodiment of the invention.

FIG. 8 is a flow diagram showing the operation of a video decoderaccording to a first embodiment of the invention.

FIG. 9 is a flow diagram showing the operation of a video decoderaccording to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Further description of the invention will be made with reference to theH.324 and H.263 recommendations. However it is not the intention tolimit the application of the invention to these or related protocols.

FIG. 1 shows a typical multimedia mobile communications system. A firstmultimedia terminal 1 communicates with a second multimedia terminal 2via a communications link 3 and a communications network 4. Control datais sent between the two terminals 1, 2 as well as multimedia data. Inthe embodiments of the invention to be described, the multimediaterminals 1, 2 are mobile/wireless videophones and the communicationsnetwork is a mobile communications network such as a GSM network. Thecommunications link 3 in this arrangement is a radio link. In otherembodiments of the invention, the multimedia terminals may both bePublic Switched Telephone Network (PSTN) videophones or one may be amobile multimedia terminal and one may be a PSTN multimedia terminal.The terminals 1, 2 may be used for real-time application such asvideo-telephony or for non-real-time applications such as retrievalservices.

FIG. 2 shows the typical multimedia components of a terminal 1 whichconforms to H.324. The terminal comprises a video codec 10 conforming toH.263, an audio codec 20 conforming to G.723.1, a data protocol manager30 conforming to T.120, a control manager 40 which outputs signalaccording to the H.245 control protocol, a multiplexer/demultiplexer 50conforming to H.223 and a modem 60 (if required). The video codec 10receives signals from a video capture device of the terminal (e.g. acamera (not shown)) for coding and receives signals from a remoteterminal 2 for decoding and display by the terminal 1 on a display 70.The audio codec 20 receives signals for coding from the microphone (notshown) of the terminal 1 and receives signals from a remote terminal 2for decoding and reproduction by a loudspeaker (not shown) of theterminal 1. These standards referred to above are described forexemplary purposes only and are not intended to be limiting.

The control manager 40 controls the operation of the video codec 10, theaudio codec 20, the data protocol manager 30 and themultiplexer/demultiplexer 50. However, since the invention is concernedwith the operation of the video codec 10, no further discussion of theother parts of the terminal will be provided.

The video codec 10 receives a digital input video signal from a signalsource (not shown). The video signal represents a sequence of frameswhere each frame is a still image. When displayed in sequence, theframes provide the impression of an image containing movement. Thus thesequence of frames are referred to herein as a moving image. The codec10 encodes the moving image from the signal source (not shown) anddecodes a received signal representing a moving image for display on thedisplay 70.

FIG. 3 shows the data structure for a frame (or picture) of a videosignal encoded according to H.263. Each frame begins with a pictureheader 80, usually of around 50 bits. The picture header 80 includes:

-   -   a Picture Start Code (PSC) for synchronisation;    -   a Temporal Reference (TR) formed by incrementing the value of TR        in the temporally-previous reference picture (e.g. I-frame)        header by one plus the number of skipped or non-reference        pictures since the previously transmitted reference picture;    -   Type Information (PTYPE) indicating, among other things, whether        the frame is an INTRA frame or an INTER frame, the format of the        picture (CIF, QCIF etc.); and    -   Quantiser information (PQUANT), which indicates the DCT        quantiser to be used for the rest of the picture.

Following the picture header 80 is picture data 82 for the first segment(GOB, slice etc.) of the picture. Owing to the presence of the pictureheader 80, a segment header for the first segment is unnecessary. Thusthe picture data 82 following the picture header 80 includes amacroblock motion vector 821 (if applicable) and block data 822.

After the data 82 for the first segment of the picture is a segmentheader 84 (e.g. GOB header) for the next segment. This GOB headerincludes:

-   -   a GOB start code (GBSC) for synchronisation;    -   a Group Number (GN) indicating the number of the GOB within the        picture;    -   GOB Frame ID (GFID) which has the same value in every segment of        a given picture and the same value as in the previously coded        picture if the two pictures are of the same type (I, P etc.);        and    -   quantiser information (GQUANT) indicating the quantiser to be        used for the rest of the picture (unless changed subsequently in        the bitstream).

The segment header 84 for the second segment is followed by the picturedata 86 (i.e. macroblock motion vector (if applicable) and block data)for that segment. The frame data continues with segment headers 84 andpicture data 86 until the whole frame has been encoded. A picture header80 for the next frame is then sent.

It will be clear to a reader that the loss of a picture header can havesevere effects on the decoding of a picture. The decoder will not beable to synchronise to the picture, will not know how the picture hasbeen encoded (I or P), etc. Conventionally, when the picture header iscorrupted, the whole of the data is discarded and a request for an INTRApicture update is sent to the transmitting device. In response, thetransmitting device codes a frame in INTRA mode and the current pictureis frozen on the display until this new INTRA-coded data is received anddecoded.

FIG. 4 shows an example of a video codec 10 according to the invention.The video codec comprises an encoder part 100 and a decoder part 200.

Considering the terminal 1 as transmitting coded video data to terminal2, the operation of the video codec 10 will now be described withreference to its encoding role. The encoder part 100 comprises an input101 for receiving a video signal from a camera or video source (notshown) of the terminal 1. A switch 102 switches the encoder between theINTRA-mode of coding and the INTER-mode.

In INTRA-mode, the video signal from the input 101 is input directly toa DCT transformer 103 which transforms the pixel data into DCTcoefficients. The DCT coefficients are then passed to a quantiser 104which quantises the coefficients. Both the switch 102 and the quantiser104 are controlled by an encoding control manager 105 of the video codecwhich also receives feedback control from the receiving terminal 2 bymeans of the H.245 control manager 40. The data output from thequantiser 104 is passed through an inverse quantiser 108 and an inverseDCT transformer 109. The resulting data is added to the contents of apicture store 107 by adder 110. In INTRA mode, the switch 115 is openedso that the contents of the picture store 107 are overwritten by theoutput of the inverse DCT transformer 109.

In INTER mode, the switch 102 is operated to accept from a subtractor106 the difference between the signal from the input 101 and a previouspicture which is stored in the picture store 107. The difference dataoutput from the subtractor 106 represents the prediction error betweenthe current picture and the previous picture stored in the picture store107. The prediction error is DCT transformed and quantised. The data inthe picture store 107 is then updated by passing the data output by thequantiser 104 through the inverse quantiser 108 and the inverse DCTtransformer 109 and adding the resulting data to the contents of thepicture store 107 by adder 110, the switch 115 being closed. A motionestimator 111 may generate motion compensation data from the data in thepicture store 107 in a conventional manner.

The video coder 100 produces header information (e.g. a temporalreference flag TR 112 a to indicate the number of the frame being coded,an INTRA/INTER flag 112 b to indicate the mode of coding performed (I orP/B), a quantising index 112 c (i.e. the details of the quantiser used),the quantised DCT coefficients 112 d and the motion vectors 112 e forthe picture being coded. These are coded and multiplexed together by thevariable length coder (VLC) 113. The output of the encoder is thenmultiplexed with other signals by multiplexer 50.

In a first embodiment of the invention, the encoder is arranged to sendrepeats of the picture header after every INTRA frame. A data store 114is therefore provided to store temporarily the data to be repeated. Inthe first embodiment, for every INTRA frame, the picture header 80 andthe first segment of data 82 are repeated for transmission to areceiving decoder. Thus the encoder outputs data in the form shown inFIG. 5.

As shown in FIG. 5, the coded signal begins with the data for the firstpicture 510 of the video signal. This frame is INTRA coded. The datacomprises the picture header 80, the data for the first segment 82 andheaders 84 and data 86 for subsequent segments of the first picture. Thepicture header 80 and the data 82 for the first segment of the firstpicture 510 are then repeated as data 512, the repeated picture headerhaving the same temporal reference TR as the original frame. Thisrepeated data is followed by data for subsequent INTER-coded frames 520,522, 524. When the next INTRA frame is coded, the data 510′ for theframe is followed by a repeat 512′ of the picture header 80 and firstsegment data 82 for the INTRA frame 510′. This arrangement leads to anoverhead of around 227 bytes per INTRA-frame for a 28.8 kbps connectionand a QCIF picture.

The receiving decoder will therefore receive a duplicate of the headerinformation. In this scenario, the decoder is arranged to operate asdescribed in Annex N of H.263 with reference to the Reference PictureSelection (RPS) mode. According to H.263 Annex N, if a decoder receivestwo or more picture headers having the same Temporal Reference (TR),then the second and subsequent picture headers (and their related data)are ignored by the decoder. Thus, if a receiving decoder manages tocorrectly decode the first occurrence of the picture header (and thusread the TR of this header), the decoder will ignore the repetition ofthe picture header. Thus an encoder according to the first embodiment ofthe invention will be operable with a conventional decoder, althoughsuch an arrangement will not result in the benefits of the invention.Compatibility is however provided.

In the first embodiment described above, the repeated data relates to anincomplete part of a frame and in particular to the picture header andthe data for the first segment of the picture. A decoder according tothe invention therefore detects the presence of repeated data bydetecting that data for an incomplete frame has been received and usesstored data to complete the frame.

In a second embodiment of an encoder according to the invention,redundant video frames are added to the encoded bit stream. Such aredundant frame is not used to bring any additional information to thetransmitted video sequence. Instead the redundant frame is used torepeat the picture header of a previous picture. The redundant framesare added to the video bitstream by an encoder according to theinvention. The presence of a redundant frame is explicitly signaled to adecoder or the decoder may use implicit characteristics of the redundantframes to detect the presence of a such redundant frame.

FIG. 6 shows the framing structure of a signal output by an encoderaccording to the second embodiment of the invention. The encoder isarranged to generate and send a redundant frame 612 after each INTRAframe 610. According to H.263, consecutive compressed pictures cannotrepresent the same uncompressed picture unless the Reference PictureSelection (RPS) mode is selected (Annex N). The second embodiment of theinvention does not rely on RPS being selected. In this case, the pictureheader only is stored in the data store 114. Under control of thecontrol 105 the original picture header 80 is altered such that the newpicture header 80′ is the same as that for the INTRA frame 610 exceptthat the picture coding type in the PTYPE field is changed from I to Pand the TR field is incremented. The control 105 also sets a field 88which indicates that there has been no change to the data for the wholeframe. In H.263 this field includes a Coded Macroblock Indication (COD)flag, which is set in respect of a macroblock that is INTER-coded and,when set, indicates that no further information is sent for themacroblock (i.e. no-change). Subsequent INTER-frames 620, 622, 624,encoded in the same manner as frames 520, 522, 524 shown in FIG. 5, aretransmitted until the next INTRA-frame 610′.

According to another embodiment of the invention, redundant frames areincluded after INTER-frames as well as INTRA-frames.

The redundant frame of repeated data 612 contains a picture header 80′of around 50 bits, 99 COD bits (one for each of the 99 macroblockswithin a QCIF picture) and some stuffing bits to make up an integernumber of bits for a complete frame. Altogether such a redundant frametypically consists of 19 bytes and thus adds around 8% of overhead tothe data stream for a 28.8 kbps H.263 connection and a QCIF picture.This overhead value applies only if each INTRA frame and each INTERframe is associated with a redundant frame. Clearly the overhead may bereduced if a redundant frame is encoded after each INTRA frame only.

As described with reference to FIGS. 5 and 6, the repeated pictureheader for a frame is provided subsequent to the original data for theframe of a picture and prior to data for the next frame.

A third embodiment of the encoder will now be described. This embodimentis based on a new addition to the Supplemental Enhancement Informationfield (Annex L) of H.263. The addition enables repetition of certainpicture layer fields of the previous picture in the supplementalenhancement information fields of the current picture. (Picture layerfields are not repeated within the same picture since they are in dangerof being corrupted at the same time as the picture layer data itself.)

The inclusion of Supplemental Enhancement Information in a pictureheader is indicated, according to H.263, by a flag PEI. If PEI is set,this indicates that supplementary information follows in an 8-bit fieldPSUPP. A further PEI indicates that a further PSUPP field follows withfurther information and so on.

Decoders which do not support the extended capabilities described inAnnex L are designed to discard PSUPP if PEI is set to 1. This enablesbackward compatibility for the extended capabilities of Annex L so thata bitstream which makes use of the extended capabilities can also beused without alteration by decoders which do not support thosecapabilities.

Annex L of H.263 describes the format of the supplemental enhancementinformation sent in the PSUPP field of the picture layer of thisRecommendation. The presence of this supplemental enhancementinformation is indicated in PEI, and an additional PEI bit is insertedbetween every octet of PSUPP data.

The PSUPP data consists of a four-bit function type indication FTYPE,followed by a four-bit parameter data size specification DSIZE, followedby DSIZE octets of function parameter data, optionally followed byanother function type indication, and so on. A decoder which receives afunction type indication which it does not support can discard thefunction parameter data for that function and then check for asubsequent function type indication which may be supported. The FTYPEvalues which have been defined are shown in Table L.1 of H.263. Thisembodiment of the invention would require some changes to Annex L ofH.263. These changes are:

-   -   1. the definition of a new function type indication (FTYPE) in        Table L.1 of H.263 e.g. Entry 13—Picture Layer Data Repetition;        and    -   2. the inclusion in Annex L of an explanation of the effect of        this FTYPE code e.g.:        -   The Picture Layer Data Repetition Function shall be used to            repeat certain fields of the coded representation of the            picture layer data of the previous picture. The repeated            fields shall appear in natural syntactic order beginning            from the Temporal Reference (TR) field. In other words, if            the PEI bits were removed from the repeated picture layer            data, the bit stream of the repetition would be exactly the            same as the original bit stream in the corresponding            position. The DSIZE field of the SEI indicates the number of            repeated bytes. A DSIZE equal to zero is reserved for future            use. The picture header information then follows the            FTYPE/DSIZE octet.

This proposed method introduces a considerable delay compared with theprevious embodiments when recovering a corrupted picture header, sincethe recovery cannot take place until the beginning of the next pictureis received. However, since the operation of a decoder is typicallyfaster than real-time video-data transmission at least at low framerates, the decoder is likely to be able to recover the time spent forwaiting the next picture to arrive.

One possible way to implement an encoder according to the thirdembodiment is presented in the flowchart shown in FIG. 7. With respectto this embodiment, picture header refers to picture layer datapreceding Supplemental Enhancement Information in the bit stream syntax.

The uncompressed signal is input to the encoder (700) at a certain framerate. A bit rate control algorithm decides whether to code or to skip aparticular frame (702). If a frame is going to be coded, the pictureheader is coded first (704). The picture header is also stored (708) indata store 114. No more than three picture headers are needed at anymoment, namely the header from the current picture and the headers fromthe two previous coded pictures. The encoder determines (706) whetherthe GFID is going to be changed in this picture (compared with theprevious picture) based on the picture headers of the current andprevious pictures. If the GFID of the previous picture also differedfrom the GFID of the picture before that (710), one needs to repeat thepicture header of the previous picture as Supplemental EnhancementInformation. Otherwise, the receiver can recover the picture header ofthe previous picture (712) using the GFID of either the current pictureor the picture preceding the previous picture. Finally, the rest of thepicture is encoded (714). Then the coding loop continues from thebeginning (700).

The repeated picture header may be repeated without the PSC.Alternatively the header could be manipulated by a systematicerror-correcting code. A systematic error-correcting code is such that kfirst symbols are the actual message and the rest of the symbols areused for error checking. In this particular case, k first bits are thepicture header, and the rest of the bits are transmitted as SupplementalEnhancement Information in the next frame. Consequently, the selectionof the error-correcting code affects how many bit inversion errors canbe detected and corrected and how many supplemental bits are needed toprovide this error protection.

In the embodiments of the encoder 100 described above the encoder hasbeen pre-programmed to send picture header repeats. However the encoder100 can be arranged additionally to repeat or refresh the picture datain response to a command from a decoder.

Additionally or alternatively the encoder may be arranged to send arepeat picture header every time the GFID parameter changes state.

Considering the terminal 1 as receiving coded video data from terminal2, the operation of the video codec 10 according to the invention willnow be described with reference to its decoding role. The terminal 1receives a multimedia signal from the transmitting terminal 2. Thedemultiplexer 50 demultiplexes the multimedia signal and passesdemultiplexed signals to the correct parts of the receiver e.g. thevideo data to the video codes 10, the audio data to the audio codec 20and H.245 control data to the H.245 control 40. The decoder 200 of thevideo codec decodes the encoded video data by inverse quantising,inverse DCT transforming and motion compensating the data. The decodedvideo data is then output for reproduction on a display 70 of thereceiving terminal 1.

As shown in FIG. 4, the decoder part 200 of the video codec 10 comprisesa variable length decoder 218, an inverse quantiser 220, an inverse DCTtransformer 221, a motion compensator 222, a picture store 223, acontroller 224, a temporary picture data store 228 and switches 230 and232. The controller 224 receives video codec control signalsdemultiplexed from the encoded multimedia stream by the demultiplexer50. In practice the controller 105 of the encoder and the controller 224of the decoder may be the same processor.

The controller 224 of the decoder checks the integrity of the receiveddata. An error in the picture header may mean that the picture cannot bedecoded and is lost completely or it is so corrupted that it iseffectively lost.

A first embodiment of the decoder will now be described. In normaloperation, the decoder 200 receives encoded data. The Variable LengthDecoder (VLD) 218 decodes the received data in an attempt to reproducethe original frame structure which has a format such as shown in FIG. 3.That is to say, the VLD 218 decompresses the encoded data and thecontroller 224 detects the Picture Start Code (PSC) within the receiveddata. The controller 224 then uses the information within the pictureheader to control the inverse quantiser 220 and the switch 230. When thePTYPE information indicates an INTRA-frame, the switch 230 is opened andthe output of inverse DCT device 221 is input to the picture store 223.When the PTYPE information indicates an INTER-frame, the switch 230 isclosed and the contents of the picture store 223 are added to the outputof the inverse DCT device 221 (the decoded prediction error) by adder234.

If the decoder is unable to decode the first picture header, but is ableto detect other segments of the picture (e.g. the GBSC of the secondsegment 84) then the decoder stores this data in the temporary picturedata store 228. When the decoder receives, decodes and identifies therepeated header data (and the first segment data 82), the decoder thenuses the data in the temporary picture store to reproduce the rest ofthe picture.

Thus, if the controller 224 does not detect a PSC at the start of aframe (or otherwise determines that the picture header is corrupted) butdoes detect a segment header (e.g. by detecting a GOB start code GBSC),the controller 224 changes the status of switch 232 such that the dataoutput from VLD 218 is input to the temporary picture data store 228.This data will start from the detected GBSC code since the VLD will notbe able to synchronise to the start of the picture.

Referring to FIG. 5, let us assume that the decoder has detected theGBSC in the header 84 for the second segment of frame 510. The datastored in the temporary picture data store 228 therefore comprisesheader 84 onwards i.e. the header for the second segment, data for thesecond segment, the header for the third segment, data for the thirdsegment etc of frame 510.

If the lost/corrupted picture header belonged to an INTRA-frame, thenext data received by the decoder will therefore be the repeated pictureheader and first segment data 512. The decoder receives the data 512relating to the repeated picture header 80 and repeated first segmentdata 82. The controller detects the PSC in the repeated data 512, readsthe PTYPE field in the header and then instructs the quantiser 220 as tothe quantiser to be used and opens switch 230 in response to the PTYPEfield in the header indicating an INTRA frame. The rest of the repeatedinformation (i.e. the repeated first segment of the data) is decoded bythe inverse quantiser 220 and the IDCT transform 221 and the decodedrepeated first picture segment is output from IDCT 222 to the picturestore 223.

The decoder recognises that the data is not for a whole picture i.e. itis only a picture header 80 followed by picture data 82 for a firstsegment followed by a picture header for a subsequent frame, by forinstance, the decoder decoding the repeated data 512 and then detectingthat the subsequent start code is for a different frame i.e. frame 520.In response to this detection by the decoder, the controller 224 altersthe status of switch 232 such that the data from frame 510 stored in thetemporary picture store 228 is output to the inverse quantiser 220 andthe IDCT transform 221. The decoded data is then output to the picturestore 223 to update the contents of the picture store with the rest ofthe decoded data for the current picture.

As mentioned above, in the first embodiment of a decoder according tothe invention, the decoder detects the receipt of a repeated pictureheader by detecting the occurrence of a picture header which is notfollowed by data for a whole picture (e.g. a picture followed by datafor one segment of the picture but no more). Other ways could be used todetect the repetition of header information.

As explained earlier, if the decoder is able to decode the frame 510correctly, the decoder simply discards the repetition of the header 512when the signal is formatted as shown in FIG. 5.

FIG. 8 shows a flow diagram illustrating a method of operating a decoderaccording to the first embodiment of the invention. Firstly (400) thedecoder 200 starts to decode a received signal by checking if a picturestart code (PSC) is the next code in the incoming data. If the pictureheader is deemed to be corrupted (402), the controller stores (404) thepicture data associated with the remaining segments of the picture inthe temporary picture data store 228.

Various ways may be used to determine if the picture is corrupted. Someexemplary methods are if the decoder cannot detect the PSC, or if anerror detection method (such as H.223 CRC checksum) indicates that thereis an error, or if an unlikely parameter is found in the picture header(e.g. an INTER flag is set within a segment header when the coding typeof the picture header is INTRA).

The decoder 200 then seeks the next error-free header. If this header isfor an INTRA frame, the decoder tries to decode the frame. If it isfound that some of the picture segments are missing, the correspondingsegments of the previous frame are read from the temporary picture store228 and decoded. If the lost/corrupted picture header belonged to anINTRA-frame, the next data received by the decoder will therefore be therepeated picture header and first segment data 512. The decoder decodes(408) the picture header and the data for the first segment of thepicture. The decoder detects (406) that the data is not for an entireframe and, in response, the decoder then decodes (408) the data storedin the temporary picture data store 228 on the basis of this repeatedpicture header.

Normal error concealment techniques may then be used to conceal errorswithin the picture which have arisen from transmission or decodingerrors. As is conventional, the decoder may also send an update requestto the encoder if the decoded frame is considered too erroneous.

A conventional decoder, on receiving an incomplete frame of data, wouldconclude that the missing data has been lost in transmission. Thus thedecoder would request an INTRA picture request in the known manner. Thusan encoder according to the invention can operate with a decoder that isnot in accordance with the invention.

A second embodiment of a decoder according to the invention will now bedescribed. With reference to a signal formatted as shown in FIG. 6, ifthe decoder is unable to decode the original header of frame 610, thedecoder stores the remaining picture data (84, 86) for the frame in thetemporary picture store 228. The first segment of the frame is notstored because it cannot be identified by the decoder. When theredundant frame 612 is received, the decoder reads the data as beingINTER coded but with no change. An encoder according to the prior artwould not usually supply this information (it being apparently 100%redundant). A decoder according to the invention detects the receipt ofa repeated picture header by detecting the occurrence of an INTERpicture header followed by a field indicating no-change. On receipt ofsuch data, the decoder uses the INTER picture header to configure thedecoder and then decodes the information from the previous frame, storedin the store 228.

In this embodiment, the data for the first segment of the picture is notrepeated and may therefore be considered to be lost. The decoder, onreceipt of the repeated header data, therefore causes the switch 232 toalter status such that the contents of the picture data are refreshedfrom the second segment onwards. Alternatively, the decoder may be ableto estimate where the first segment picture data should begin in thecorrupted data and decode the data from that point. For instance, let usassume that there is a one bit inversion error in the picture header ofthe original picture and therefore the picture header cannot be decoded.However the PSC is still valid and the start of the frame can thereforebe detected reliably. The whole picture 610 is therefore stored in thetemporary picture store 228 and then when the repeated header isreceived, the decoder 200 starts to decode the stored data at theposition where the picture header is expected to end and where the datafor the first segment is expected to begin.

Thus, the decoder inspects the incoming data. If the picture header islost or corrupted, the data for the remainder of the frame is stored inthe temporary picture data store 228. Subsequent data is then decodedand if the data relates to an INTER-frame and indicates that there is nochange in the picture, the picture header is decoded and the data fromthe temporary picture data store 228 is decoded using the information inthe picture header of the redundant frame.

When the signal is formatted as shown in FIG. 6, if the decoder managesto correctly decode the picture header of frame 610, the decoder willcontinue and decode the repetition of the header 612. As described withreference to FIG. 6, the repeated information 612 comprises the pictureheader 82 (including an incremented TR) and a field 88 indicating thatnone of the data has changed with respect to the previous coded frame.Since there is no data stored in the temporary picture data store 228,the decoder will discard the redundant frame of data 612 and decode thesubsequent frame 620.

On receipt of a signal encoded according to the third embodiment of theinvention, a decoder according to the invention uses the data followingthe FTYPE/DSIZE octet of the Supplemental Enhancement Information in thesubsequent frame to decode the data stored in the temporary picturestore 228.

The third embodiment of the decoder will now be described with referenceto FIG. 9. This embodiment makes use of the SEI method as describedearlier with reference to the encoder and FIG. 7.

The decoder operates as follows. At first (900), the decoder receivesthe picture header of the next transmitted picture. If the header isfree from errors (901), the decoder can decode the header withoutproblems (902). Then, it can continue to decode the rest of the picture(904). If some errors were detected in the picture header (901), thedecoder searches (906) for the first error-free picture segment (GOB orslice) header of the picture. Let us call this bit stream position asthe first resynchronisation position. If the GFID of that header is thesame as in the previous picture (908), the decoder can recover thecrucial parts of the picture header (910) and continue decoding (904),starting from that particular picture segment. If the GFID differs fromthe one in the previous picture (908), the decoder searches (912) forthe next picture start code. If the picture layer data of that picturecontains SEI picture header repetition (914), the decoder can recoverthe picture header of the current picture (916). It must also set thedecoding position in the bit stream back to the first resynchronisationposition (918). If the picture layer data does not contain SEI pictureheader repetition, the decoder searches for the next picture segmentstart code and checks (920) if the GFID in the header is the same as theGFID of the picture that is being decoded. If the GFIDs are equal, thedecoder can recover the picture header (910) and continue decoding fromthe first resynchronisation position. If the GFIDs are different fromeach other, the decoder has no means to recover the corrupted pictureheader. In this case (922), it can request for an INTRA update, forexample.

The temporary picture store may store coded data for a plurality offrames. Since most frames in low bit rate applications are coded in anINTER frame manner, most of the data stored in the temporary picturedata store is likely to represent prediction error data and hence berelatively compact. The temporary picture store therefore should besufficient to store data for at least one INTRA frame and one INTERframe of data, an INTER frame typically being coded with around 250bytes for a QCIF picture at 28.8 kbps.

If any data for subsequent frames of the video are also stored in thetemporary-picture data store 228, these are also decoded and output tothe picture store 223 to bring the contents of the picture store 223into alignment with the contents of the corresponding picture store ofthe transmitting device.

1. A method of video source encoding, using an apparatus, the methodcomprising: generating, using the apparatus, a first source encodedbit-stream portion representative of a first video frame, the firstsource encoded bit-stream portion comprising picture header data for thefirst video frame and corresponding source encoded picture data for thewhole of the first video frame; generating, using the apparatus, asecond source encoded bit-stream portion, the second source encodedbit-stream portion comprising a repeat of at least a part of the pictureheader data for the first video frame; and providing, using theapparatus, the second source encoded bit-stream portion in the sourceencoded bit-stream subsequent to and outside the first source encodedbit-stream portion without repeating the corresponding source encodedpicture data for the whole of the first video frame.
 2. A methodaccording to claim 1, comprising generating, using the apparatus, asecond source encoded bit-stream portion comprising a repeat of at leasta part of the picture header data for the first video frame only if thefirst video frame is encoded in INTRA-frame format.
 3. A methodaccording to claim 1, comprising generating, using the apparatus, thesecond source encoded bit-stream portion as a source encoded bit-streamportion representative of an incomplete part of a video frame that onlycomprises a repeat of at least a part of the picture header data for thefirst video frame and a first segment of the source encoded picture datafor the first video frame.
 4. A method according to claim 1, comprisinggenerating, using the apparatus, the second source encoded bit-streamportion as a source encoded bit-stream portion that comprises a repeatof at least a part of the picture header data for the first video frameand further data, the further data indicating that the second sourceencoded bit-stream portion is representative of a video frame that isunaltered with respect to a reference frame.
 5. A method according toclaim 4, wherein the further data comprises an indication or indicationsthat no source encoded picture data has changed with respect to aprevious video frame.
 6. A method according to claim 1, comprisinggenerating, using the apparatus, the second source encoded bit-streamportion as a source encoded bit-stream portion representative of a videoframe subsequent to the first video frame in encoding order, the secondsource encoded bit-stream portion comprising picture header data for thesubsequent video frame and source encoded picture data for thesubsequent video frame, and providing, using the apparatus, a repeat ofat least a part of the picture header data for the first video frame inthe picture header data of the subsequent video frame.
 7. A methodaccording to claim 6, comprising repeating, using the apparatus, onlycertain fields of the picture header data for the first video frame inthe picture header data of the subsequent video frame.
 8. A methodaccording to claim 1, comprising generating, using the apparatus, thesecond source encoded bit-stream portion as a source encoded bit-streamportion representative of a video frame subsequent to the first videoframe in encoding order, the second source encoded bit-stream portioncomprising picture header data for the subsequent video frame and sourceencoded picture data for the subsequent video frame and providing, usingthe apparatus, a repeat of at least a part of the picture header datafor the first video frame in Supplemental Enhancement Information of thesubsequent video frame.
 9. A method according to claim 8, comprisingproviding, using the apparatus, a repeat of at least a part of thepicture header data for the first video frame in the SupplementalEnhancement Information of the subsequent video frame, excluding thepicture start code for the first video frame.
 10. A method according toclaim 1, comprising generating the second source encoded bit-streamportion as a source encoded bit-stream portion representative of anincomplete part of a video frame that only comprises the picture headerdata for the first video frame and a first segment of the source encodedpicture data for the first video frame and identifying the second sourceencoded bit-stream portion as comprising a repeat of the picture headerdata for the first video frame by giving the repeated picture headerdata the same temporal reference as the picture header data of the firstvideo frame.
 11. Apparatus for video source encoding, wherein theapparatus is configured to: generate a first source encoded bit-streamportion representative of a first video frame, the first source encodedbit-stream portion comprising picture header data for the first videoframe and corresponding source encoded picture data for the whole of thefirst video frame; generate a second source encoded bit-stream portion,the second source encoded bit-stream portion comprising a repeat of atleast a part of the picture header data for the first video frame; andprovide the second source encoded bit-stream portion in the sourceencoded bit-stream subsequent to and outside the first source encodedbit-stream portion without a repeat of the corresponding source encodedpicture data for the whole of the first video frame.
 12. Apparatus forvideo source encoding according to claim 11, wherein the apparatus isconfigured to generate a second source encoded bit-stream portioncomprising a repeat of at least a part of the picture header data forthe first video frame only if the first video frame is encoded inINTRA-frame format.
 13. Apparatus for video source encoding according toclaim 11, wherein the apparatus is configured to generate the secondsource encoded bit-stream portion as a source encoded bit-stream portionrepresentative of an incomplete part of a video frame that onlycomprises a repeat of at least part of the picture header data for thefirst video frame and a first segment of the source encoded picture datafor the first video frame.
 14. Apparatus for video source encodingaccording to claim 11, wherein the apparatus is configured to generatethe second source encoded bit-stream portion as a source encodedbit-stream portion that comprises a repeat of at least part of thepicture header data for the first video frame and further data, thefurther data indicating that the second source encoded bit-streamportion is representative of a video frame that is unaltered withrespect to a reference frame.
 15. Apparatus for video source encodingaccording to claim 14, wherein the further data comprises an indicationor indications that no source encoded picture data has changed withrespect to a previous video frame.
 16. Apparatus for video sourceencoding according to claim 11, wherein the apparatus is configured togenerate the second source encoded bit-stream portion as a sourceencoded bit-stream portion representative of a video frame subsequent tothe first video frame in encoding order, the second source encodedbit-stream portion comprising picture header data for the subsequentvideo frame and source encoded picture data for the subsequent videoframe and to provide a repeat of at least a part of the picture headerdata for the first video frame in the picture header data for thesubsequent video frame.
 17. Apparatus for video source encodingaccording to claim 16, wherein the apparatus is configured to repeatonly certain fields of the picture header data for the first video framein the picture header data of the subsequent video frame.
 18. Apparatusfor video source encoding according to claim 11, wherein the apparatusis configured to generate the second source encoded bit-stream portionas a source encoded bit-stream portion representative of a video framesubsequent to the first video frame in encoding order, the second sourceencoded bit-stream portion comprising picture header data for thesubsequent video frame and source encoded picture data for thesubsequent video frame and to provide a repeat of at least a part of thepicture header data for the first video frame in SupplementalEnhancement Information of the subsequent video frame.
 19. Apparatus forvideo source encoding according to claim 18, wherein the apparatus isconfigured to provide a repeat of at least a part of the picture headerdata for the first video frame in the Supplemental EnhancementInformation of the subsequent video frame, excluding the picture startcode for the first video frame.
 20. Apparatus for video source encodingaccording to claim 11, wherein the apparatus is configured to generatethe second source encoded bit-stream portion as a source encodedbit-stream portion representative of an incomplete part of a video framethat only comprises the picture header data for the first video frameand a first segment of the source encoded picture data for the firstvideo frame and to identify the second source encoded bitstream portionas comprising a repeat of the picture header data for the first videoframe by giving the repeated picture header data the same temporalreference as the picture header data of the first video frame.
 21. Anon-transitory computer readable medium comprising a source encodedbit-stream, the source encoded bit-stream comprising: a first sourceencoded bit-stream portion representative of a first video frame, thefirst source encoded bit-stream portion comprising picture header datafor the first video frame and corresponding source encoded picture datafor the whole of the first video frame; and a second source encodedbit-stream portion, the second source encoded bit-stream portioncomprising a repeat of at least a part of the picture header data forthe first video frame, the second source encoded bit-stream portionprovided in the source encoded bit-stream subsequent to and outside thefirst source encoded bit-stream portion without a repeat of thecorresponding source encoded picture data for the whole of the firstvideo frame.
 22. A network device comprising apparatus for video sourceencoding, wherein the apparatus for video source encoding is configuredto: generate a first source encoded bit-stream portion representative ofa first video frame, the first source encoded bit-stream portioncomprising picture header data for the first video frame andcorresponding source encoded picture data for the whole of the firstvideo frame; generate a second source encoded bit-stream portion, thesecond source encoded bit-stream portion comprising a repeat of at leasta part of the picture header data for the first video frame; and providethe second source encoded bit-stream portion in the source encodedbit-stream subsequent to and outside the first source encoded bit-streamportion without a repeat of the corresponding source encoded picturedata for the whole of the first video frame.
 23. A network comprising anetwork device according to claim
 22. 24. A terminal device comprisingapparatus for video source encoding, wherein the apparatus for videosource encoding is configured to: generate a first source encodedbit-stream portion representative of a first video frame, the firstsource encoded bit-stream portion comprising picture header data for thefirst video frame and corresponding source encoded picture data for thewhole of the first video frame; generate a second source encodedbit-stream portion, the second source encoded bit-stream portioncomprising a repeat of at least a part of the picture header data forthe first video frame; and provide the second source encoded bit-streamportion in the source encoded bit-stream subsequent to and outside thefirst source encoded bit-stream portion without a repeat of thecorresponding source encoded picture data for the whole of the firstvideo frame.
 25. An encoder configured to: generate a first sourceencoded bit-stream portion representative of a first video frame, thefirst source encoded bit-stream portion comprising picture header datafor the first video frame and corresponding source encoded picture datafor the whole of the first video frame; generate a second source encodedbit-stream portion, the second source encoded bit-stream portioncomprising a repeat of at least a part of the picture header data forthe first video frame; and provide the second source encoded bit-streamportion in the source encoded bit-stream subsequent to and outside thefirst source encoded bit-stream portion without a repeat of thecorresponding source encoded picture data for the whole of the firstvideo frame.
 26. Apparatus for video source encoding comprising: meansfor generating a first source encoded bit-stream portion representativeof a first video frame, the first source encoded bit-stream portioncomprising picture header data for the first video frame andcorresponding source encoded picture data for the whole of the firstvideo frame; means for generating a second source encoded bit-streamportion, the second source encoded bit-stream portion comprising arepeat of at least a part of the picture header data for the first videoframe; and means for providing the second source encoded bit-streamportion in the source encoded bit-stream subsequent to and outside thefirst source encoded bit-stream portion without a repeat of thecorresponding source encoded picture data for the whole of the firstvideo frame.
 27. Apparatus for video source encoding, comprising acontroller configured to: generate a first source encoded bit-streamportion representative of a first video frame, the first source encodedbit-stream portion comprising picture header data for the first videoframe and corresponding source encoded picture data for the whole of thefirst video frame; generate a second source encoded bit-stream portion,the second source encoded bit-stream portion comprising a repeat of atleast a part of the picture header data for the first video frame; andprovide the second source encoded bit-stream portion in the sourceencoded bit-stream subsequent to and outside the first source encodedbit-stream portion without a repeat of the corresponding source encodedpicture data for the whole of the first video frame.
 28. A method ofvideo source encoding, using an apparatus, the method comprising:generating, using the apparatus, a first source encoded bit-streamportion representative of a first video frame, the first source encodedbit-stream portion comprising picture header data for the first videoframe and corresponding source encoded picture data for the whole of thefirst video frame; generating, using the apparatus, a second sourceencoded bit-stream portion representative of a second video framesubsequent to the first video frame in encoding order, the second sourceencoded bit-stream portion comprising picture header data for the secondvideo frame, picture data for the whole of the second video frame and arepeat of at least a part of the picture header data for the first videoframe; and providing, using the apparatus, the second source encodedbit-stream portion in the source encoded bit-stream subsequent to thefirst source encoded bit-stream portion.
 29. A method according to claim28, comprising providing, using the apparatus, a repeat of at least apart of the picture header data for the first video frame in the pictureheader data of the subsequent video frame.
 30. A method according toclaim 29, comprising repeating, using the apparatus, only certain fieldsof the picture header data for the first video frame in the pictureheader data of the subsequent video frame.
 31. A method according toclaim 28, comprising providing, using the apparatus., a repeat of atleast a part of the picture header data for the first video frame inSupplemental Enhancement Information of the subsequent video frame. 32.A method according to claim 31, comprising providing, using theapparatus, a repeat of at least a part of the picture header data forthe first video frame in the Supplemental Enhancement Information of thesubsequent video frame, excluding the picture start code for the firstvideo frame.
 33. An encoder configured to: generate a first sourceencoded bit-stream portion representative of a first video frame, thefirst source encoded bit-stream portion comprising picture header datafor the first video frame and corresponding source encoded picture datafor the whole of the first video frame; generate a second source encodedbit-stream portion representative of a second video frame subsequent tothe first video frame in encoding order, the second source encodedbitstream portion comprising picture header data for the second videoframe, picture data for the whole of the second video frame and a repeatof at least a part of the picture header data for the first video frame;and provide the second source encoded bit-stream portion in the sourceencoded bit-stream subsequent to the first source encoded bit-streamportion.
 34. An encoder according to claim 33, wherein the encoder isconfigured to provide a repeat of at least a part of the picture headerdata for the first video frame in the picture header data for thesubsequent video frame.
 35. An encoder according to claim 34, whereinthe encoder is configured to repeat only certain fields of the pictureheader data for the first video frame in the picture header data of thesubsequent video frame.
 36. An encoder according to claim 33, whereinthe encoder is configured to provide a repeat of at least a part of thepicture header data for the first video frame in SupplementalEnhancement Information of the subsequent video frame.
 37. An encoderaccording to claim 36, wherein the encoder is configured to provide arepeat of at least a part of the picture header data for the first videoframe in the Supplemental Enhancement Information of the subsequentvideo frame, excluding the picture start code for the first video frame.38. An encoder comprising: means for generating a first source encodedbit-stream portion representative of a first video frame, the firstsource encoded bit-stream portion comprising picture header data for thefirst video frame and corresponding source encoded picture data for thewhole of the first video frame; means for generating a second sourceencoded bit-stream portion representative of a second video framesubsequent to the first video frame in encoding order, the second sourceencoded bit-stream portion comprising picture header data for the secondvideo frame, picture data for the whole of the second video frame and arepeat of at least a part of the picture header data for the firstframe; and means for providing the second source encoded bit-streamportion in the source encoded bit-stream subsequent to the first sourceencoded bit-stream portion.
 39. An encoder comprising a controllerconfigured to: generate a first source encoded bit-stream portionrepresentative of a first video frame, the first source encodedbit-stream portion comprising picture header data for the first videoframe and corresponding source encoded picture data for the whole of thefirst video frame; generate a second source encoded bit-stream portionrepresentative of a second video frame subsequent to the first videoframe in encoding order, the second source encoded bit-stream portioncomprising picture header data for the second video frame, picture datafor the whole of the second video frame and a repeat of at least a partof the picture header data for the first video frame; and provide thesecond source encoded bit-stream portion in the source encodedbit-stream subsequent to the first source encoded bit-stream portion.40. A method of video source encoding using an apparatus, the methodcomprising: generating, using the apparatus, a first source encodedbit-stream portion representative of a first video frame, the firstsource encoded bit-stream portion comprising picture header data for thefirst video frame and corresponding source encoded picture data for thewhole of the first video frame; applying, using the apparatus an errorcorrecting code to the picture header data for the first video frame togenerate error correction data; generating, using the apparatus, asecond source encoded bit-stream portion representative of a secondvideo frame subsequent to the first video frame in encoding order, thesecond source encoded bit-stream portion comprising picture header datafor the second video frame and picture data for the whole of the secondvideo frame; and providing, using the apparatus, the error correctiondata in Supplemental Enhancement Information of the second video frame.41. An encoder configured to: generate a first source encoded bit-streamportion representative of a first video frame, the first source encodedbit-stream portion comprising picture header data for the first videoframe and corresponding source encoded picture data for the whole of thefirst video frame; apply an error correcting code to the picture headerdata for the first video frame to generate error correction data;generate a second source encoded bit-stream portion representative of asecond video frame subsequent to the first video frame in encodingorder, the second source encoded bitstream portion comprising pictureheader data for the second video frame and picture data for the whole ofthe second video frame; and provide the error correction data inSupplemental Enhancement Information of the second video frame.